This invention relates to an apparatus for the production of a middle layer of chipboards (i.e. particle boards) or the like and/or for the manufacture of chipboards consisting essentially only of one middle layer, with at least one spreading station associated with a spreading or stewing chamber arranged above a moving support, such as an endless chip layer carrier or the like, for the reception of a chip layer (i.e. particle layer) or the like, to which the entire fine and coarse material serving for the formation of a type of middle layer can be fed via at least one metering tank.
Actually, two methods are utilized for the production of chipboards or the like, namely the so-called throw spreading process and primarily the air screening process. Both methods are based on the idea of applying the material to be used for the formation of a chip layer or the like to a layer carrier in the sifted condition, in order to obtain in the zone of the topside and underside of the boards to be manufactured a maximally fine to dust-like material, whereas the topside layer is followed in the downward direction by fine, rather coarse, coarse, rather coarse, and fine particles. Thus, normally coarse material is present in the middle layer (DAS [German Published Application] 1,228,404). If the so-called middle layer is relatively thick, the material for this middle layer is optionally also fed to the layer carrier in the unsifted state. Also, measures have been taken, namely by the arrangement of additional spreading or strewing stations, to include also finer components in the rather coarse and/or coarse material to fill up any present cavities with these fine particles (DAS 1,109,867). This increases the transverse tensile strength.
The glue-covered components available for the formation of a chipboard normally consist of various fractions.
First fraction: PA0 Second fraction:
[a] Larger, thinner particles having a width of 2-25 mm., a length of 10-25 mm., and a thickness of 0.2-0.5 mm. PA1 [b] Larger, thicker particles having the same external dimensions as indicated under (a), but with a thickness of 0.5-2 mm. PA1 and thereabove. PA1 [a] Smaller, thicker particles (granular). PA1 [b] Smaller, thinner particles (dust and fibers).
In general, it is desirable to manufacture chipboards having a maximally fine, closed surface. To attain this objective, dust and fibrous particles (second fraction [b]) are to be brought to the surface. This can be readily effected most advantageously by means of conventional air screening stations which bring dust, as well as fine and fibrous particles, to the surface of the layer to be compressed into a board. Moreover, the chipboards or the like are to exhibit good mechanical properties, i.e. they must not only have a sufficient transverse tensile strength, but also an adequate bending strength [flexural strength]. Therefore, a kind of middle layer is provided between the two outer layers, namely a mixture of rather large, thicker particles (first fraction [b]) and smaller, thicker particles (second fraction [a]) wherein the smaller, thicker particles fill out the spaces between the larger, thicker particles, which enhances the transverse tensile strength.
The flexural tensile strength can be increased by providing larger, thinner particles (first fraction [a]) respectively underneath or above the fine surface layers, for example at least underneath the upper, fine-particle surface layer.